1. Field
This disclosure relates generally to semiconductor device packaging, and more specifically, to flat no-leads packages that provide reliable automatic optical inspection cues after the semiconductor device package is soldered to a printed circuit board or other carrier in second level assembly.
2. Related Art
Space concerns in assembling semiconductor devices has led to development of semiconductor device packages that have no leads extending from beyond the perimeter of the package. These packages, such as quad-flat no-leads (QFN and pQFN) packages and dual-flat no-leads (DFN and uDFN) packages, are surface mounted to printed circuit boards (PCBs). The lack of leads extending from the package results in a mounted package consuming less space on the PCB and therefore permitting higher density mounting of packages on the PCB.
The surface mount process used for mounting no-leads packages on a PCB typically uses a solder reflow process that electrically and mechanically bonds the contacts on a no-leads package to contacts on the PCB. Surface tension of the solder to portions of the contacts that have an affinity for solder results in some of the solder extending beyond the edge of the mounted component along the PCB contact, and rising along the edge of the contact of the mounted component away from the PCB. This forms a “toe fillet” that can be automatically optically inspected as an aid in determining whether the contacts of the package and the PCB are bonded.
In a typical no-leads package, the electrical contacts are present on the bottom (or active) surface of the package. To enhance the contacts' affinity for solder, the contacts are often plated with a solderable metal (e.g., gold, silver, nickel, tin, and alloys thereof). During singulation, a portion of an electrical contact is exposed along the side of the package, but most of this exposed portion of the electrical contacts is not plated, but instead may be exposed copper that is subject to oxidization and may also have a rougher surface due to the singulation process. Thus, there may be a lesser affinity for solder on the portion of the contact along the side of the package. This lesser affinity can affect the formation of a toe fillet, and thereby cause the bonded package to fail optical inspection even though the bond of the package to the PCB may be adequate along the active surface of the package. It is therefore desirable to provide a no-leads package that consistently provides for well-formed toe fillets so that bonded parts do not wrongly fail an optical inspection process.
The use of the same reference symbols in different drawings indicates identical items unless otherwise noted. The figures are not necessarily drawn to scale.